In a heat pump air conditioner, frost may be formed on a heat exchanger of an outdoor unit during a heating operation. There exists a heat pump air conditioner that performs a defrost operation for removing frost formed on a heat exchanger of an outdoor unit during a heating operation. During this defrost operation, it is a general practice to have a refrigerant flow in a reverse direction as compared to the heating operation, such that defrosting is performed by heat radiated when a high-temperature and high-pressure gas refrigerant discharged from a compressor condenses at the heat exchanger of the outdoor unit.
Patent Literature 1 discusses a configuration of an expansion mechanism such that a flow path resistance during a defrost operation is lower than a flow path resistance during a normal cooling or heating operation. With this arrangement, a sufficient flow volume of the refrigerant can be secured for melting frost, so that an efficient defrost operation is realized and a defrost operation time is shortened.
Patent Literature 2 discusses a defrost operation in a water heater. In particular, Patent Literature 2 discusses adjustment of a circulation amount of a refrigerant during the defrost operation based on the temperature of water, i.e. a heat source. With this arrangement, efficiency is enhanced by shortening a defrost operation time depending on the temperature of water, and reliability is enhanced by preventing a liquid backflow to a compressor. A liquid backflow means that the refrigerant in a liquid phase flows into the compressor.